CODEWITHSUNDEEP
pythonregex
manadart
manadart

Reputation: 1810

Regular Expression for Combined Look-ahead/Look-behind

I'm using python and trying to write a regular expression that matches a hyphen (-) if it is not preceded by a period (.) and not followed by one character and a period.

This one is matching hyphen not preceded by a period and not followed by a character:

r'(?<!\.)(-(?![a-zA-Z]))'

Nothing I've tried seems to get me the right match for the negative look-ahead part (single character and period).

Any help appreciated. Even a totally different regex if I'm barking up the wrong tree altogether.

Edit

Thanks for the answers. I did actually try 

r'(?<!\.)(-(?![a-zA-Z]\.))'

But I now realise that my logic was wrong, not my expression.

I've chosen the answer and upvoted the other correct ones :)

Upvotes: 1

Views: 1018

Answers (2)

eyquem
eyquem

Reputation: 27585

import re

ss = '  a-bc1    d-e.2    .-gh3    .-N.4'
print 'The analysed string:\n',ss

print '\n(?!\.-[a-zA-Z]\.)'
print 'NOT (preceded by a dot AND followed by character-and-dot)'
r = re.compile('(?!\.-[a-zA-Z]\.).-...')
print r.findall(ss)

print '\n(?<!\.)-(?![a-zA-Z]\.)'
print 'NOT (preceded by a dot OR followed by character-and-dot)'
q = re.compile('.(?<!\.)-(?![a-zA-Z]\.)...')
print q.findall(ss)

result

The analysed string:
  a-bc1    d-e.2    .-gh3    .-N.4

(?!\.-[a-zA-Z]\.)
NOT (preceded by a dot AND followed by character-and-dot)
['a-bc1', 'd-e.2', '.-gh3']

(?<!\.)-(?![a-zA-Z]\.)
NOT (preceded by a dot OR followed by character-and-dot)
['a-bc1']

Which case do you want in fact ?

Upvotes: 1

Justin O Barber
Justin O Barber

Reputation: 11601

Assuming that by "character" you mean (and I base this assumption on your example and on @SimonO101's comment) [A-Za-z], I think you are looking for something like this:

>>> r = re.compile(r'(?<!\.)-(?![A-Za-z]\.)')
>>> r.search('k.-kj')
>>> r.search('k-l.')
>>> r.search('k-ll')
<_sre.SRE_Match object at 0x02D46758>
>>> r.search('k-.l')
<_sre.SRE_Match object at 0x02D46720>
>>> r.search('l-..')
<_sre.SRE_Match object at 0x02D46758>

There is no need to try to enclose the hyphen in a group that also captures the negative lookahead assertion. Trying to do this only complicates the matter.

Upvotes: 3

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